I think it depends on what plants and other things you have. I had one of those long gold ones that was about 1,500 years old when I sold it. I was using that one and a black one to make the yellow expensive magic fish, but I only had one of the long shiny gold ones, so I just kept it and used it over and over.

I dont know the max years, I also think it depends on the type of fish, some seem to have more health than others when you click to see their health and if they need to eat.